Recent research pertaining to (a) high frequency audiometry up to 20 kHz (b) to the measurement of in-the-ear pressures produced by hearing aids and (c) to the research and diagnostic applications of so-called cochlear distortion products have necessitated accurate knowlege of the acoustic wave motion in the external meatus. The acoustic wave motion within the external meatus is a function of frequency, of the size (length, crossectional shape) of the canal, of the physical properties of the eardrum, of the eardrum inclination re. earcanal axis, of the location and size of the sound source, and of the size, shape, location, and acoustic impedance of the microphone to be used for the pressure measurement in the earcanal. The proposed "Phase I" research is to collect a set of molds of the external meati and of the conchae of human cadavers in order to determine the range of sizes and shapes of the canals and conchae and the inclination of the eardrum relative to the ear-canal axis. The physical dimensions and shapes of these earmolds will be measured in order to design some scaled models which have acoustically significant but yet realistic features. Initially, miniature microphones will be used to determine the acoustical significance or order of importance of each "feature", such as curvature of the canal, eccentricity of the crossection, eardrum inclination relative to the canal axis, off-axis placement of the source, etc. Then, specific microphone designs will be tested which will either cause a minimum of interference with the sound pressure at the microphone's location and at the eardrum or cause a systematic predictable deviation from the true pressure so that a proper correction can be made. Such microphones, might be of a segmented, annular type or of a multielement circumferential type, for example. Theoretical models will be used as a guide for such microphone designs in terms of shapes and placement and for the prediction of sound pressures at the "eardrum". The long range goals for Phase II are to develop experimental and theoretical models for sound sources, microphones, and wave-propagation models in the external ear which extend to the high frequencies. The goal of Phase III is to produce such instrumentation commercially.